The present invention relates to a static pressure adjustment device for an exhaust duct receiving heated gas from a gas-fired burner or the like through a heat exchanger, wherein the burner must be turned off when the gas flow rate falls below a predetermined minimum.
Gas-fired burners or heaters are conventionally used to heat or dry components or assemblies in an oven or chamber in industrial applications. In many such applications, the oven or heating chamber includes a combustible gas. For example, a paint drying oven such as used by the automotive industry may include volatile organic compounds (vocs), such as solvents, which are highly combustible. In such applications, the heated gas from the burner is directed to a heat exchanger which indirectly heats the oven or heated chamber and the heated gas is then vented through an exhaust duct. It is essential, however, that the gas-fired burner be turned off in the event that the gas flow volume from the heat exchanger through the exhaust duct falls below a predetermined minimum rate or volume to avoid a hazardous condition.
In this type of application, the exhaust duct from the heat exchanger generally includes a fixed orifice plate having a central opening. A switch having pressure sensors on opposed sides of the fixed orifice plate then turns the gas-fired burner off in the event that the gas flow through the exhaust duct falls below a predetermined minimum volume. As will be understood, the sensors of the safety switch measure the fluid pressure drop across the fixed orifice plate. The National Fire Protection Association (NFPA) Code requires that the burner be shut off in the event that duct gas (air) is not flowing through the exhaust to prevent a hazardous condition.
The principal problem with such fixed orifice plates in an application of the type described occurs during start up. For example, the minimum pressure drop required across a conventional fixed orifice plate may be 0.2 inches of water. This exact condition is difficult to reach, particularly during start up prior to balancing the system. It would therefore be desirable to be able to adjust the opening through the fixed orifice plate particularly during start up of the oven. Once the opening through the fixed orifice plate is adjusted, generally no further adjustment will be required following installation.
The problems associated with startup of a modern paint drying oven, for example, are compounded by the fact that an automotive paint drying oven may have 8 to 20 heat zones ranging in temperature from about 300xc2x0 F. to 600xc2x0 F. Each zone of the oven includes a gas fired burner which directs heated gas to a heat exchanger which indirectly heats the zone. It is not possible to simply calculate the pressure drop required to actuate the switch shutting off the burner for each zone because the density of the air circulated through the oven decreases as the temperature increases. Therefore, it is conventional to start up the oven and heat each of the zones to the required temperature and measure the pressure drop across the fixed orifice plate using the switch. Then, the fixed orifice plates are replaced as required with orifice plates having a greater or smaller opening. This is a time consuming task. There is therefore a need for a simple method of adjusting the opening through the orifice plate which may be maintained after start up.
As set forth above, the present invention relates to a static pressure adjustment device particularly, but not exclusively, for adjusting the opening through a fixed orifice plate in an exhaust duct communicating with a gas-fired burner through a heat exchanger providing indirect heat to a drying oven or heated chamber containing combustible gas.
As described, the exhaust duct receiving heated gas from a gas-fired burner or the like includes a fixed annular orifice plate extending generally perpendicular to the direction of gas flow having a periphery fixed to an inside surface of the duct and a central opening for maintaining a predetermined minimum fluid pressure drop at a predetermined gas flow rate through the exhaust duct. In a typical application, the exhaust duct is cylindrical and welded at the seam making it difficult to adjust the opening or orifice through the fixed annular orifice plate. A switch mechanism is provided having a pressure sensor on opposed sides of the fixed annular orifice plate which turns the burner off when the pressure drop across the fixed orifice plate drops below a predetermined minimum.
The exhaust duct system of this invention further includes a moveable orifice adjustment plate mounted adjacent the fixed orifice plate at an upstream side of the fixed annular orifice plate having an opening generally aligned with the opening through the fixed annular orifice plate and the exhaust duct system further includes an adjustment mechanism connected to the moveable orifice adjustment plate for moving the moveable orifice adjustment plate relative to the fixed orifice plate from a retracted position, wherein the opening of the moveable orifice adjustment plate is generally aligned with the opening through the fixed annular orifice plate, to an extended position restricting the opening through the fixed annular orifice plate to adjust the effective opening through the fixed and moveable orifice plates thereby adjusting the pressure drop across the orifice plates.
In the preferred embodiment, wherein the exhaust duct is cylindrical as described, the opening through the fixed annular orifice plate is circular and the opening of the moveable orifice plate is generally semicircular. In the most preferred embodiment, the moveable orifice adjustment plate is generally C-shaped including a generally semicircular opening having a radius generally equal to the radius of the opening through the fixed annular orifice plate, such that when the moveable orifice adjustment plate is in the retracted position, the opening through the fixed annular plate is not restricted by the moveable orifice adjustment plate. Further, in the preferred embodiment, the adjustment mechanism includes an adjustment element or rod connected to the moveable orifice adjustment plate and extending through the exhaust duct to permit adjustment of the position of the moveable orifice adjustment plate from outside the duct.
In the disclosed embodiment, the adjustment device includes a rod connected to the mid-portion of the C-shaped moveable orifice adjustment plate, such as by welding, which extends through the wall of the exhaust duct and including an externally threaded portion which is threadably received in an internally threaded members used as jam nuts to lock the moveable orifice adjustment plate in the desired location. The position of the moveable orifice adjustment plate relative to the fixed annular orifice plate may then be adjusted simply by releasing the jam nuts and moving the rod.
The static pressure adjustment device for an exhaust duct of this invention therefore solves the problem of adjusting the orifice through a fixed orifice plate in a simple and reliable manner. Other advantages and meritorious features of this invention will be more fully understood from the following description of the preferred embodiments, the appended claims and the drawings, a brief description of which follows.